1. Field of the Invention
The present invention relates to an apparatus for stacking signatures; more particularly, the present invention relates to a vertical signature stacking system having a means to control stack formation based on a non-contact sensing of the reaction of the feed means to the forming stack.
2. Description of Related Art
There is disclosed in the art a wide variety of horizontal and vertical stacking machines. These machines are designed to stack sheets of paper, including "signatures". Typically, the signatures have a major plane and a perimeter and are stacked with the major planes of adjacent signatures in contact with the signature perimeters aligned. The signatures are usually rectangular shaped as they are fed from a printer or folding machine. Signatures are fed at a high rate of speed to the stacker, by a conveyor belt assembly.
Horizontal stackers stack the signatures with each individual signature in a vertical position supported on an edge. The stack has a stack axis which is horizontal. Vertical stackers have a vertical stacking support which supports the major plane of the signatures. The term "vertical" is nominal, and used to indicate that the stack is formed with the stack axis vertical or at an angle to the horizontal, typically greater than 45.degree. and most commonly at 45.degree. to 75.degree..
Vertical stackers comprise a vertical stacking table having a vertical support wall. The vertical signature support wall extends from a feed end to a discharge end. The vertical signature support is nominally vertically, but typically at an angle of from 1.degree. to 45.degree. to the vertical. In this way the forming stack rests against the vertical support wall. The vertical stacking table has a sliding signature support fork which can translate up and down along the vertical support wall. The signature support fork can be transverse to and extending from the vertical support track. A stack is squared between the sliding signature support fork and the vertical signature support wall. There can be a feed fork assembly which supports the oncoming signatures as the stack begins to form. The signatures are fed to the stacker by a conveyor belt assembly. A useful conveyor belt assembly is shown in U.S. Ser. No. 07/688,039 filed Apr. 19, 1991 and hereby incorporated by reference. The conveyor belt assembly comprises a lower supporting feed conveyor and an upper feed conveyor. The signatures are fed to the stacker from between the lower and upper conveyors. The forming stack is transferred from the feed fork to the primary fork. The formed stack on the primary support fork is removed to a bundling and strapping apparatus where the stack is compressed and strapped to form a bundle.
In forming the bundles it is desirable to have end boards or end plates at either longitudinal end of the stack. Apparatus to automatically insert end boards during the stacking operation are disclosed in U.S. Pat. Nos. 4,372,201; 4,311,090; 4,554,867; and 4,772,003.
During operation of a vertical stacker, the signatures are fed to the stacker at a rate that is determined by operation of the system. The rate at which the support forks descend is set based on the expected signature stream speed. A continuing concern during the forming of stacks at high speeds is the correlation of the descending fork speed with the signature speed to assure process control and well formed stacks. It is undesirable for any reason for the stack to be forming faster or slower than the support fork is descending. If for example, the signatures are slightly thicker than expected the stack will be higher at the stacker and blockage of the system can occur. If the signatures are thinner than expected, the upper feed conveyor cannot precisely place each oncoming signature on the stack and a poorly formed stack can result. In order to avoid this concern the position of the upper feed conveyor belt which extends above the support forks is measured. This has heretofore been done by attempting to measure belt location, or tension. Such mechanical sensing and control means has been limited in solving this problem.